Walls – How to paint inside corners of textured walls at the ceiling or baseboard

Taking things in reverse order:

With the large areas firstly remove any remaining loose plaster. Trying to patch without doing this is a waste of time. Then with the large areas you'll need to replaster. You can either take this as an opportunity to learn or pay someone. I won't tell you how to plaster as it's really a skill you have to be shown or practise yourself, though I do have a couple of bits of advice. Don't try to plaster the whole wall in one go. Take it in stages and make sure you dampen the wall you're plastering onto as it helps the plaster stick.

With the areas missing the top coat of plaster you will need to reskim. This should be done after fixing the base coat. You might find a plasterer willing to skim over your base coat - discuss it before you start. This will give you a good finish but should cost less than hiring a plasterer to do the whole job. Getting a good skim finish takes a lot of practice.

Finally the areas with only minor cracks and bumps should be filled and sanded before proceeding.

If you manage to get a good surface then painting is the best option. If not then paper the wall with two layers of lining paper. The first runs horizontal and the second vertical. This will smooth out most of the remaining unevenness in the wall. Before papering wash the walls with a weak glue solution this will stop all the glue from the paper being absorbed into the plaster and the paper lifting off the wall.

I should have added that you need to leave the plaster to fully dry out before painting or papering. That might have been one of the reasons why the previous repair has failed.

UPDATE

The quote you have doesn't seem unreasonable but without knowing the exact area you need replastering or your location it's difficult to say for certain. However, we shouldn't be getting into commenting on exact quotes as they are always going to be too localised (in time and space). Get another quote to compare it against - that's always going to be the best option. Also as I said above see if you can get the plasterer just to do the final skim - it should come out cheaper.

I never tape anything and people are amazed at the crisp lines I paint in my home. I use a high quality angled brush for this. Depending on my wrist fatigue and room, I work from either left to right, or vice versa with this technique. This is self-taught and I have no idea if their is a name for this.

I load the brush up with plenty of paint and then spread the paint on about an inch lower than the ceiling, onto the wall for about the length of a foot. This is just to get the bulk of the paint onto the wall to prevent dripping and to help load the brush back up while I work. I pull the brush along the wall just below the ceiling and slowly raise it up to the corner so that the bristles fall in to place. Without the angled brush, this would be difficult. I hold the brush more on plane with the ceiling than the wall, almost perpendicular to the wall. Once the bristles have fallen into a nice setup, most people would simply drag the brush and paint the wall with a one long stroke. I found that I run out of paint and have to use multiple shallow arcs to do this. This was unsatisfactory to me. I wiggle the brush back and forth while dragging it along to drag and push the paint to the place I want. The key of this method is to control the amount of paint loaded up on the brush at the ends while you drag and push. With the push and pull action of this method, I think it would work wonderfully for a textured surface as it pushes paint with both strokes in each direction

Upon very close inspection, the finished edge will have a small ragged appearance to it, but you can only see it when you are very close up. From normal viewing distance, even in a small bathroom, the line appears to be a crisp edge, well-defining the corner. I will be painting a room in the next week or two. I could take a quick video and post if it would be beneficial to anyone.

Edited to add video:

Video